Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction.

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn ^III ₃ Mn ^IV O ₃ (CH ₃ COO) ₃ (A-α-SiW ₉ O ₃₄ )] ^6- . Theoretical results indicated that catalytic active species 1 (Mn ₄ ^{III,III,IV,IV} ) was formed after O ₂ formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn ^IV -oxyl radical 4 (Mn ₄ ^IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn ₂ O ₂ core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn ^IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn ₄ ^III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol ^-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn ₄ ^III,IV,IV,IV -O ₂ ⋅) enabled the O ₂ release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.
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